1. Field of the Invention
This invention pertains broadly to the field of telecommunications. More particularly, the invention pertains to underwater communications. In still greater particularity, the invention relates to underwater acoustic communications in multipath environments.
2. Description of the Prior Art
As acoustic engineers know, underwater acoustic communication in shallow operating depths is often hampered by what is known as multipath. Multipath disrupts transmission and reception of acoustic signals sent underwater.
This phenomenon can be generally explained as follows: acoustic propagations from an underwater transmission point may travel directly to an underwater receiver yet, these propagations often reflect from the ocean surface and ocean floor to create cancellation and distortion of the directly propagated transmission. Signal cancellation and distortion due to multipath has been minimized by the use of directional receivers. Further, frequency diversity transmission and reception has been utilized.
In this latter method, transmission of identical signals is done simultaneously over a number of different or diverse frequencies. Depending upon the multipath structure of the environment through which the signals are propagated, some of the transmission frequencies will be more vulnerable to being cancelled or distorted by multipath. Transmitting the same message simultaneously over a broad spectrum of frequency diverse channels improves chances of clear communication, as one or more of the transmitted signals may ultimately be received without severe multipath disruption.
Methods of transmitting a message simultaneously over a number of frequency diverse channels are not new. Yet, heretofore, these methods were not readily adaptable to the environment in which they were used. Simultaneous redundant transmissions appreciably slow data transmission rates. If the efficiency of underwater acoustic communications could be readily observed, an operator could make efforts to tailor a system to provide data rate transmittals commensurate with the multipath environment.